Heavy industrial equipment is often powered by larger displacement combustion engines ranging between 5 L (305 c.i.d.) and 300 L (18,300 c.i.d.) or more. Large displacement combustion engines generally require the use of engine starters coupled to the engine by a starter drive assembly to start. Starter drive assemblies use an inertia drive mechanism (“inertia drive’) to transfer rotation from the starter motor's output shaft to the flywheel of the combustion engine for initial engine startup.
Typically, the inertia drive includes a clutch plate stack coupled to the output shaft of the starter motor and a screw shaft on which a pinion gear rides. The clutch plate stack is pressed together by a number of coiled pressure springs. During engine startup, the turbine air motor driven from the source of air/gas turns its output shaft. The inertia drive transfers the rotation of the starter motor's output shaft via the clutch plate stack to drive the screw shaft. The inertia of the pinion gear causes it to be translated along the screw shaft and into engagement with a ring gear of the engine. Once the pinion gear reaches the end of its travel along the screw shaft, it is fully meshed with the engine's ring gear. Continued rotation of the screw shaft rotates the pinion gear, which in turn rotates the ring gear of the engine to start the engine. Once the engine starts, it begins to accelerate the ring gear faster than the rotation of the screw shaft. This results in the pinion gear being translated along the screw shaft away from and out of engagement with the ring gear.
The basic design of inertia drives has remained unchanged since the 1960s. Heretofore, the design of inertia drives has required costly manufacturing methods in order to achieve the desired functionality. By way of example, the acme threads formed in the screw shaft and pinion must be precisely machined so that the contact surfaces of the clutch housing and pinion properly orient and align for driving engagement. Consequently, the acme threads must be formed using a thread grinder in order to ensure the proper tolerances and orientation between the components. Grinding the acme threads into the screw shaft is a more expensive and time consuming manufacturing method. In addition, wave springs and other components have been incorporated into the design, as well as reinforced in order to accommodate and control the extreme torques generated from increasing more powerful starter motors. Nevertheless, inertia drives are often damaged from the impact forces transmitted from the extreme torque loads instantly generated from conventional hydraulic starter motors.